In diesel engines, fuel is injected into a combustion chamber through a fuel injection valve and compressed and ignited after a predetermined time (an ignition delay) since fuel injection. To improve output performance and emission performance in a diesel engine, a control device for controlling an engine control parameter such as a fuel injection timing or amount is generally employed with the ignition delay taken into consideration.
The lower the cetane number of fuel, the longer the ignition delay of a diesel engine becomes. Accordingly, if relatively low-cetane fuel such as winter fuel is supplied to a fuel tank in a diesel engine having an engine control parameter that has been set for fuel with a standard cetane number before shipment, the ignition timing of the fuel is delayed and the state of fuel combustion is unfavorable. In some cases, misfire may happen.
To prevent such a problem, it is desirable to correct the engine control parameter based on the actual cetane number of fuel injected into a combustion chamber. For effective correction of the parameter, accurate estimation of the cetane number of the fuel is necessary.
Conventionally, as described in Patent Document 1, a control device for a diesel engine that injects a small amount of fuel from a fuel injection valve and estimates the cetane number of the fuel based on engine torque (output torque) generated through such fuel injection has been proposed. In the control device, the amount of fuel injected from the fuel injection valve of the diesel engine (the fuel injection amount) and the corresponding output torque are detected. With reference to the relationship between the fuel injection amount and the output torque, the cetane number of the fuel is estimated. The control device calculates the fuel injection amount based on fuel pressure detected by a pressure sensor and a variation waveform of the detected fuel pressure. The output torque produced through the fuel injection is calculated using a changing manner of the rotating speed of the output shaft of the diesel engine (the engine speed).